1. Field of the Invention
This invention relates to fluid delivery apparatus including a flow line for delivery of fluid through the flow line from a source to an outlet, with flow control means, for controlling the delivery of the fluid, being connected in the flow line.
The fluid may be in the form of a liquid, gas, or vapor, or a mixture of any one or more of these. However, the invention is especially applicable to use with liquids, and, among these, those liquids which have a boiling point below ambient temperatures.
2. Discussion of the Prior Art
Liquefied gas is widely used in research in industry and in medicine, including for example liquid oxygen, nitrogen, carbon dioxide, helium, or air. These cooled liquids are usually dispensed as needed from large vessels into smaller containers or other apparatus, through a flow line in the form of a hose or pipe, often with one or more in-line taps or valves to control the flow. The operation of such taps or valves may be automatic, that is to say they may be controlled electronically, electrically, or mechanically. In conventional practice, a single solenoid valve is placed in series in the fluid flow line to enable the flow to be initiated or shut off as required. It can easily be seen that the consequences of failure of such a valve may be serious; though relatively rare, such failures, in particular failure to close, provide real hazards to operators, as well as to materials being handled.
The catastrophic consequences of valve failure can be mitigated by the use of two valves in series instead of only a single valve. This is a well known technique in the gas industry, in which the valves are typically solenoid valves. However, failure of one of the valves can remain undetected until the other valve fails, because until this happens the failure of the first valve is masked by the one still operating.